The more I’m using shared layers, the more I believe there’s no real difference between ‘Share Layer To All Pages’ and master pages. Except for layer sharing being way more flexible, because you can put a shared layer on top of all the others.
Are there any benefits in using a master page?
I think master pages existed first and then shared layers were added for more flexibility. But I think you are right, that you can achieve everything you need to with a shared layer that you could do with a Master page + added flexibility.